Interpretive Summary: Extensive diversity among sheep breeds, production environments, and management systems necessitates systematic and comprehensive breed evaluations to allow producers to identify breeds that are best suited for their breeding objectives. Nearly 50% of the US breeding sheep inventory is located in the 11 westernmost states where sheep are primarily grazed on rangelands, but comparative breed data are lacking in these environments. Therefore, a three-year study was conducted at the U.S. Sheep Experiment Station to comprehensively evaluate Columbia, Suffolk, USMARC-Composite, and Texel breeds as terminal sires in extensive rangeland production. This report describes the reproductive performance of adult Rambouillet ewes when mated to rams of these four breeds, and the growth and survival of the resulting crossbred lambs until weaning. Breed of ram did not affect ewe reproductive performance. Breed of sire affected crossbred lamb growth to weaning, but not survival. The use of Suffolk rams to improve preweaning growth of market lambs is warranted in extensive production systems using adult Rambouillet ewes.

Technical Abstract:
A 3-yr study was conducted to comprehensively evaluate Columbia, Suffolk, USMARC-Composite (Composite), and Texel breeds as terminal sires in an extensive rangeland production system. The objective was to estimate breed-of-ram effects on ewe fertility, prolificacy, and dystocia, and sire breed effects on lamb survival and growth until weaning at approximately 132 d of age. Data were from 22 Columbia, 22 Composite, 21 Suffolk, and 17 Texel rams with 957 exposures to 574 adult Rambouillet ewes (3 to 7 yr old at lambing), 908 lambings, and 1,834 lambs. Ram breed did not affect ewe fertility (mean = 94.9%; P = 0.73), total number born per ewe lambing (mean = 2.02 lambs; P = 0.20), number born alive per ewe lambing (mean = 1.90 lambs; P = 0.24), or number weaned per ewe lambing (mean = 1.45 lambs, P = 0.94). Dystocia rates were different (P = 0.01) for ewes mated to Columbia (12.2%), Composite (13.5%), Suffolk (25.7%), and Texel rams (31.9%) during 1 yr of the study, but differences among ram breeds were not repeatable (P = 0.38) during the other 2 yr. Suffolk-sired lambs were heavier (P = 0.02) at birth (5.5 kg) and weaning (40.3 kg) than lambs sired by the other breeds, which did not differ (P = 0.34) for birth weight (mean = 5.3 kg). Texel-sired lambs (37.4 kg) were lighter (P = 0.02) at weaning than Columbia- (38.8 kg) and Composite-sired (38.4 kg) lambs, which did not differ (P = 0.40) for weaning weight. Sire breed effect approached significance (P = 0.06) for lamb survival to weaning; estimated survival probabilities were 0.87 (Columbia), 0.89 (Composite), 0.93 (Suffolk), and 0.86 (Texel) for lambs reared by their birth dam. Interaction between sire breeds and birth weight affected (P < 0.001) lamb survival and revealed that lightweight Columbia- and Suffolk-sired lambs had a greater risk of death than lightweight lambs sired by Composite and Texel rams, but risk of death did not increase substantially for heavyweight lambs from any of the breeds. When mated to adult Rambouillet ewes in an extensive rangeland production system, the use of Suffolk rams is warranted to improve preweaning growth of market lambs and is not predicted to affect ewe fertility, ewe prolificacy, dystocia, or lamb survival compared with the other sire breeds we tested.